1. Field of the Invention
The present invention relates to a liquid crystal display device and, more specifically, to a liquid crystal display device in which a pair of insulating substrates are opposed to each other via a predetermined gap that is maintained by spacers, a liquid crystal composition (a liquid crystal molecule) is held in the gap, and a storage capacitor portion is formed in each pixel region.
2. Description of the Related Art
High-resolution liquid crystal display devices capable of color display for use in notebook-sized computers and computer monitors are now widely utilized.
Basically, in this type of liquid crystal display device, what is called a liquid crystal panel is formed by holding a layer of a liquid crystal composition between two insulating substrates (hereinafter also referred to simply as substrates) such as glass plates at least one of which is transparent. This type of liquid crystal display device is generally classified into a type (simple matrix type) in which an image is formed by changing the orientation directions of liquid crystal molecules of desired pixels by selectively applying voltages to various electrodes for pixel formation that are formed on the insulating substrates of the liquid crystal panel and a type (active matrix type) in which various electrodes for pixel formation and active elements for pixel selection are formed and an image is formed by changing the orientation directions of liquid crystal molecules of desired pixels by making selections from the active elements.
In general, the active matrix liquid crystal display device employs that is called a vertical electric field type in which electric fields are developed between electrodes formed on one substrate and an electrode formed on the other substrate.
On the other hand, the liquid crystal display device of what is called a lateral electric field type (also called as In-Plane Switching type, abbreviated as “IPS type” hereinafter) has been put into practical use in which the directions of electric fields that act on the liquid crystal layer are approximately parallel with the substrate surfaces. In an example of the lateral electric field type liquid crystal display device, a very wide viewing angle is obtained by forming comb-teeth electrodes for electric field formation on one of the two substrates.
In the lateral electric field liquid crystal display device, an active matrix substrate is provided with scanning signal lines and video signal lines, switching elements formed in the vicinity of the crossing points of the scanning signal lines and the video signal lines, pixel electrodes to which drive voltages are applied via the respective switching elements, and counter electrodes that are formed in the same plane as the pixel electrodes. A color filter substrate is provided with a black matrix made of a resin composition and color filter layers formed for each pixel in an opening region of the black matrix. A liquid crystal panel is formed by holding a liquid crystal composition between the active matrix substrate and the color filter substrate. The liquid crystal display device is configured in such a manner that a backlight is disposed in the rear of the liquid crystal panel and a unified structure is obtained by using top and bottom cases.
Image display is performed by changing the light transmittance of the liquid crystal compound by electric field components that are formed between the pixel electrodes and the counter electrodes so as to extend approximately parallel with the substrate surfaces.
In contrast to the vertical electric field type one, the lateral electric field type liquid crystal display device is superior in viewing angle; that is, it allows a user to view a clear image even when he is located at a position that forms a large angle with the display screen.
The liquid crystal display device having the above configuration is disclosed in Japanese Unexamined Patent Publication No. Hei. 6-160878 and its counterpart U.S. Pat. Nos. 5,598,285, and 5,737,051, for example.
FIG. 1 is a plan view showing one pixel, a light shield region of a black matrix BM, and its vicinity of a conventional lateral electric field type liquid crystal display device.
As shown in FIG. 1, each pixel is provided in a region enclosed by four signal lines that cross each other, that is, a scanning signal line (gate signal line or horizontal signal line) GL, a counter voltage signal line CL, and two adjacent video signal lines (drain signal line or vertical signal line DL.
Each pixel includes a thin-film transistor TFT, a storage capacitor portion Cstg, a pixel electrode PX, and an counter electrode CT. In FIG. 1, a plurality of scanning signal lines GL and counter voltage signal lines CL are provided at the top and bottom of the pixel direction so as to extend in the right-left or horizontal direction. A plurality of video signal lines DL are provided at the right-left side of the pixel so as to extend in the top-bottom or vertical direction. The pixel electrode PX is connected to the thin-film transistor TFT, and the counter electrode CT is integral with the counter voltage signal line CL.
The pixel electrode PX and the counter electrode CT confront each other, and display is controlled by modulating transmission light or reflection light by controlling the orientation state of a layer of a liquid crystal composition LC (hereinafter also referred to simply as a liquid crystal or a liquid crystal layer) by means of an electric field developed between the pixel electrode PX and the counter electrode CT. Each of the pixel electrode PX and the counter electrode CT assumes a comb-teeth shape and has long and narrow portions extending in the top-bottom or vertical direction in FIG. 1.
The pixel electrode PX and the counter electrode CT are formed in such a manner that the number P of comb-teeth portions of the pixel electrode PX and number C of comb-teeth portions of the counter electrode CT in one pixel necessarily satisfy a relationship C=P+1 (in FIG. 1, C=2 and P=1). The comb-teeth portions of the counter electrode CT and those of the pixel electrode PX are arranged alternately so as to have the comb-teeth portions of the counter electrode CT arranged adjacent to the video signal lines DL. With this structure, shielding from electric lines of force originating from the video signal lines DL can be effected by the counter electrode CT so that electric fields between the counter electrode CT and the pixel electrode PX are not influenced by the electric fields originating from the video signal lines.
The potential of the counter electrode CT is stable because it is always supplied with a potential externally via the counter voltage signal line CL. Therefore, the counter electrode CT has almost no potential variation even if it is adjacent to the video signal lines DL. Further, the above structure makes the geometrical position of the pixel electrode PX more distant from the video signal lines DL, whereby the parasitic capacitances between the pixel electrode PX and the video signal lines DL are greatly reduced and hence a variation of a pixel electrode potential Vs due to video signal voltages can be controlled.
As a result, vertically extending crosstalk lines (an image quality failure called vertical smears) can be prevented.
In a specific construction, widths Wp and Wc of the pixel electrode PX and the counter electrode CT, respectively, are set at 6 μm, which is sufficiently larger than 4.5 μm which is the maximum setting thickness of a liquid crystal layer (described later). It is desirable that the electrode widths Wp and Wc be sufficiently larger than 5.4 μm because it is preferable to provide a margin of 20% or more in view of processing variations in manufacture. As a result, electric field components parallel with the substrate surfaces that are applied to the liquid crystal layer become stronger than those perpendicular to the substrate surfaces, which prevents voltages for driving the liquid crystal to become unduly high. It is preferable that the maximum values of the electrode widths Wp and Wc be smaller than an interval L between the pixel electrode PX and the counter electrode CT. This is because it the interval between the electrodes is too short, electric lines of force are curved sharply and hence regions where electric field components parallel with the substrate surfaces are stronger than those perpendicular to the substrate surfaces are made larger, as a result of which the electric field components parallel with the substrate surfaces cannot be applied to the liquid crystal layer efficiently. To give a margin of 20% to the interval L between the pixel electrode PX and the counter electrode CT, it is necessary that the interval L be larger than 7.2 μm. For example, in a case of constructing a liquid crystal display device having a diagonal size of about 14.5 cm (5.7 inches) and a resolution of 640×480 dots, an interval L that is larger than 7.2 μm can be realized by setting the pixel pitch at about 60 μm and dividing each pixel into two parts.
To avoid disconnection, the electrode width of the video signal lines DL is set at 8 μm, which is somewhat larger than the widths of the pixel electrode PX and the counter electrode CT. To avoid short-circuiting, an interval of about 1 μm is provided between the video signal lines DL and the counter electrode CT. The video signal lines DL and the counter electrode CT are provided in different layers by forming the video signal lines DL and the counter electrode CT above and below a gate insulating film, respectively. On the other hand, the interval between the pixel electrode PX and the counter electrode CT is changed in accordance with the liquid crystal material used, for the following reason. The electric field intensity for attaining the maximum transmittance depends on the liquid crystal material. To obtain the maximum transmittance within the range of the maximum amplitude of a signal voltage that is set by the breakdown voltage of a video signal driver circuit (signal-side driver) used, the electrode interval needs to be set in accordance with the liquid crystal material. The electrode interval becomes about 15 μm when a liquid crystal material that will be described later is used.
In the example configuration being discussed, in a plan view of FIG. 1, a black matrix BM surrounds an opening of the pixel and is formed on the gate line GL, the counter voltage signal line CL, the thin-film transistor TFT, and the drain lines DL, and between the counter electrode CT and the drain lines DL. The storage capacitor portion Cstg is located outside the opening of the black matrix BM (i.e., outside the pixel region) and is composed of the pixel electrode PX, the counter voltage signal line CL, and an insulating film formed between them.
In the liquid crystal display device, an alignment film is applied after formation of the respective electrodes and electrode wiring lines, protective films, and insulating films and is given a liquid crystal alignment control ability by being subjected to a treatment called rubbing.
In the conventional lateral electric field type liquid crystal display device, since the storage capacitor portion Cstg is formed outside each pixel region, there is no large height change or steps in the pixel region and hence the alignment film in the pixel region can be given a uniform liquid crystal alignment control ability.
However, in recent years, liquid crystal display devices have been proposed in which the aperture ratio of the entire screen is increased by forming the storage capacitor portion Cstg in each pixel region. In those devices, the storage capacitor Cstg produces large steps in the pixel region and those steps may cause an alignment defect in a rubbing treatment. As a result, what is called a domain occurs and causes display unevenness.
In particular, alignment defects of the above kind occur frequently in a case where the multilayered film structure that constitutes the storage capacitor portion Cstg has steps extending perpendicularly or approximately perpendicularly to the alignment direction (rubbing direction) of the alignment film. When such an alignment defect occurs, the liquid crystal does not operate normally in the vicinity of the storage capacitor portion Cstg, to cause a domain. This results in a problem that the contrast is lowered and display unevenness occurs, which means a marked reduction in image quality.